This invention relates to a catalytic process for the sequential hydrotreating (demetalation and/or desulfurization) and mild hydrocracking of residua to provide distillate, i.e., a liquid product containing a major proportion by weight of components boiling at or below 650.degree. F.
The term "residuum" refers to a complex hydrocarbon mixture containing a large fraction boiling at or above 650.degree. F. Residua may be derived from petroleum, shale oil, tar sands or oil sands bitumen, diatomite bitumen, and coal liquids.
Motor fuel, diesel fuel and jet fuel are for the most part the most valuable products obtained from petroleum. Consequently the petroleum industry is geared to produce maximum amounts of these products. To this end, crude petroleum is distilled to obtain these desired fractions and that portion of the distillate boiling above the desired fractions is subjected to hydrocracking or to catalytic cracking to convert it to lower boiling material. However, residual petroleum oil fractions are characterized by relatively high metals and sulfur content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally precludes their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas and hydrogen. Also, the metals can irreversibly poison catalysts by blocking active sites.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction which contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants limit the coke's use to low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical, and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than that used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for environmental reasons.
A number of catalytic processes for effecting the demetalation and/or desulfurization of residua are known.
U.S. Pat. No. 3,730,879 discloses a two-bed catalytic process for the hydrodesulfurization of crude oil or a reduced fraction in which at least 50 percent of the total pore volume of the first-bed catalyst consists of pores in the 100-200 Angstrom (A) average diameter range.
U.S. Pat. No. 3,830,720 discloses a two-bed catalytic process for hydrocracking and hydrodesulfurization of residual oils in which a small pore catalyst is disposed upstream of a large-pore catalyst.
U.S. Pat. Nos. 3,696,027 and 4,054,508 each describes a three-bed catalytic hydrotreating process.
U.S. Pat. Nos. 3,876,523 and 4,082,695 each describes a process for catalyticaly demetalizing and desulfurizing hydrocarbon oils comprising residual fractions utilizing a catalyst comprising a hydrogenation component, such as cobalt and molybdenum oxides, composited on a refractory base such as alumina.